It is common to store water in unpressurized above-ground vessels from which water from time to time is drawn by means of pump or gravity flow, particularly where the water supply is intermittent, or where the instantaneous demand exceeds the flow rate of the source of supply. It is particularly desirable, to have a vessel which is sealed against egress or ingress of liquids when potable water is being stored.
The present invention is concerned with plastic tanks that principally are intended for use when oriented as vertical cylinders, including tanks which are much larger than just mentioned. For instance, plastic tanks may have capacities of in the range of about 9,500 liters to about 11,000 liters or more. A representative tank may have a diameter of about 8 feet (2.4 meters), a height of about 8 to 10 feet (2.4 to 3.1 meters), a basic wall thickness of 0.19 to 0.4 inches (4.8 to 10 mm) and a weight of about 250 to 400 pounds (114 to 180) kilograms. If made in one piece, such tanks are difficult and costly to transport because of their bulk. When such tanks are shipped as two nested bucket-like half-tank parts which are welded to each other at or near the point of use that can be a good solution.
Tanks comprised of mating half tanks which can be nested for shipment and assembled in proximity to the point of use have been made. For example, see U.S. Pat. No. 5,878,907 of Graf and U.S. Pat. No. 8,740,005 of Holbrook et al. Such tanks have a mid-elevation joint which is alternatively secured by means of clamps or by means of welding.
A related commonly owned patent application bearing Ser. No. 15/413,219, filed on Jan. 23, 2017 and entitled “Making a plastic tank with a weld joint that enables weld verification,” describes how such a tank can be fabricated by welding together two molded bucket-like parts at a circumferential flange.
Because they are large and injection molded, forming such tanks can present unusual welding problems. Because the tank parts being mated are large, they can be subject to slight manufacturing variations (e.g., those related to variations in cooling of hot molded plastic), and possible distortion during handling or temporary storage or uneven sunlight heating, etc. Injection molding is a preferred way of making tank parts because it enables precise wall thickness control. That provides efficient structural use of plastic, compared to parts made with processes such as blow molding and rotational molding that are commonly used to make large plastic structures. But the lightness and thinness of the structures and their large size can lead to unacceptable variation in the weld joint gap, where the fusion weld element might typically be less than 0.2 inch in diameter. The present invention is concerned with the manner in which tank flanges are welded to each other; in particular, it includes the use of certain kinds of clamping. Generally, it is known to clamp things during welding. The present invention employs a relatively uncommon means of fusion welding the flanges of two tank parts: the fusion welding element has means for heating contained within itself, such as a resistance wire or a conductor that is susceptible to heating due to electromagnetically induced eddy currents. The elements include those commercially referred to as PowerCore™ welding rod and Emabond™ preform for use with an Emabond electromagnetic welding system. Those products are described further below.